Muscles in the human body converge into sinew like structures (tendons) that then insert into bone. Generally this mechanism is applied across a fulcrum or joint.
In many cases, the lever arm or fulcrum involved is very short, yet the load is high. Therefore the pull that is exerted on tendons can well be several times the body weight.
This mechanism above pertains to many regions of the body, and its repair can be challenging. Ideally joints should be kept moving whilst the repair takes effect. However it can take several months for the repair process to have enough strength for normal use.
The shoulder is a more unusual example of the above because some of the muscle-tendon units are cardinal to supplying the fulcrum, not only torque. In the shoulder, the socket portion is flat and does not as much restrain the humeral head by virtue of the shape, as for sockets elsewhere in the body. The series of muscles immediately adjacent to the joint, called the rotator cuff, wrap round the humeral head and by contracting provide a type of soft tissue socket.
Musculo tendinous units may fail in several ways. Amongst these, and in the shoulder in particular, degenerative tearing of the rotator cuff tendons is common, particularly in middle age and above.
A second way in which the units fail is by fracture of the bones to which the tendons attach. The common presentation of this was described by Charles Neer as 3 and 4 part fractures of the shoulder. Such fractures may be repaired by fixation of the fragments—particularly in the younger age groups.
Sometimes and particularly in the elderly, the bone quality is so poor that fixation of the fragments and fractured humeral head is unlikely to succeed, as the screws will have no hold. In these cases the humeral head may be excised and replaced with a hemi arthroplasty (a replacement of only the ball portion of the humeral head).
A conventional prosthesis comprises a head and a stem, and the head of the prosthesis corresponds in shape to the humeral head, in particular the cartilage surface of the humeral head. The stem of the prosthesis is inserted into the top of the humerus shaft, and it is known to provide connection points at the top of the prosthesis stem which enable the greater and lesser tuberosities to be secured to the prosthesis.
The tuberosities are conventionally secured using sutures which are connected to the interface between the tuberosity bone (which has been fractured and come away from the humeral head) and the attached tendon.
There are a number of different ways to secure the tuberosities to the prosthesis. Generally, these methods use a combination of cerclage connections (in which a suture coupled to the tuberosity is secured around or to the shaft of the prosthesis) and longitudinal connections (in which a suture extends down the humerus from the tuberosity at the humerus head to a connection point on the humeral shaft).
A problem with the known shoulder replacement prostheses is that the positioning of the muscles and tendons in the replacement shoulder does not replicate the anatomical configuration. In other words it is difficult to position the tuberosities anatomically, as reference points for this are scant. Furthermore, movement of the tuberosities is often not sufficiently inhibited, and with rehabilitation they lose position and displace so that the tendon to joint connection is lost and the stability to the humeral head and therefore its fulcrum is lost. This leads to loss of active motion called pseudoparalysis and loss of strength.
Thus, repair of the tuberosity fragments and their attached rotator cuff tendons is difficult. Failure of this important element is common, and the result of these problems is pain or discomfort to the patient and/or a lack of mobility in the joint.
WO 2008/020161 discloses an improved shoulder replacement prosthesis which can more reliably replicate the anatomical structure. The head of the shoulder replacement prosthesis has a domed portion and a flange positioned at an end region of the domed portion. The flange has a width which varies around the domed portion, having a greater width at a part corresponding to a region of the greater tuberosity than at a part corresponding to a region of the lesser tuberosity, such that a rim of the flange away from the domed portion represents the position of the anatomical tendinous insertion points of the rotator cuff on the greater and lesser tuberosities. The flange is provided with a plurality of connection portions and associated surface area for securing the tendons attached to the tuberosities.
By dimensioning the flange appropriately, the anatomical positioning of the tendon to bone connections can be more accurately replicated. In particular, the flange acts to bridge the so-called bare area of the humeral head (the space between the cartilage surface and the greater tuberosity), an area which is not easily accounted for in conventional prostheses which purely supply a replica of the humeral cartilaginous head. The flange provides a firm immovable surface area to which connections can be made, so that the length of connections can be reduced, and the scope for movement of the tuberosities in the replacement joint is reduced.
Compared to more conventional suture systems, the length over which restraining connections are required for the tuberosities is reduced. With sutures, such lengthy spans of connection are prone to loosening and therefore failure.
The system above improves the anatomical replication but the efficacy of the connections still has room for improvement, so that there is still a need to improve the tendon connection mechanism.